People, vehicles, chemical process facilities and many manufacturing operations are vulnerable to hazards produced by explosions. The source of explosions may be a munition intended to inflict damage and injury or may be fuel or dust released in an accident. Regardless of the cause, explosions arising from rapid combustion processes generate shock waves, intense heat, and gas whose pressure significantly exceeds the ambient condition.
Many materials, structures, methods and other inventions have been developed that offer some protection against undesirable effects created by explosions. Most of these inventions are in the form of armor or barriers that isolate the blast from people or spaces requiring protection. Armor and barriers are typically used to protect vehicle and building interiors exposed to external explosions.
For explosions occurring outdoors, another protective measure is using components to deflect blasts away from objects. This technique does not work for confined environments. Blast protection for internal explosions typically involves venting. The existing art does not generally provide protection of people for intense blasts in confined environments, with or without venting.
Design of blast protection structures generally must consider characteristics of the explosive threat. Choice of materials, type of protective measure, and structural components also depends upon whether or not a need exists for the protective element to remain intact following an explosion. Even when all of the essential considerations are made, weight, space and geometrical constraints often render current technologies inadequate. This is particularly the case for intense blast environments.
Examples of the latter include internal spaces within aircraft, containers with explosives inside, tunnels, and corridors of buildings. The inadequacy of the current art becomes more apparent as explosive charge weight of the threat increases. The number of vehicles and buildings destroyed with large explosive charges over the last decade have vividly demonstrated the shortcomings of the present art.
Another inadequacy of the present art is inability to defend against a type of munition referred to as a shaped charge. Heavy, bulky armor assemblies using the current art are required to prevent penetration of metal jets produced by shaped charge devices. There are many versions of shaped charge devices, including ones generally termed “explosion-formed penetrators” or “EFPs”.
All versions of shaped charge munitions utilize an explosive with a thin metal lining on the charge surface facing the intended target. Detonation of the charge converts the metal lining into a projectile capable of penetrating deeply into any material or armor.
When the penetrator pierces armor, intense shock waves and hot blast gas follow through the hole formed by the metal slug. This is because most shaped charge devices detonate in close proximity to the target. These blast hazards generally inflict serious injury to people in an enclosed space such as a vehicle interior behind the pierced armor, including traumatic brain injury. The large number of casualties caused by EFPs and other shaped charge devices in recent conflicts illustrates yet another example of conventional approaches failing to provide adequate protection. Because of the widespread exposure of people and structures to many types of explosion hazards, there are many potential users who would welcome new materials and other inventions that could provide desired protection against specified blast threats with significantly less weight and with thickness no greater than required with armors of the present art. This includes practical means of reducing behind-armor blast effects caused by shaped charge munitions.
Developing improved methods of protecting against blasts and explosively formed projectiles requires consideration of all associated hazard phenomena. These hazards are described as follows.
Blast Wave Phenomenology Involving Solid Explosives
Hot gas produced by an explosion will expand rapidly. This expansion, along with rapid heating, will accelerate the molecules comprising air in the surrounding space. Localized acceleration of gas molecules creates pressure above ambient, often called “overpressure”.
By definition, the compression process of explosions occurs faster than the acoustic speed of ambient air, thereby generating shock waves that propagate away from the blast. A blast event thus comprises an initial shock wave, followed by an accelerated gas pulse, then by formation of a hot gas cloud at elevated pressure (with debris if near the ground).
Explosion parameters such as pressure, impulse (momentum transfer), temperature, and shock wave pressure duration are strongly affected by interaction with objects interacting with a blast wave. Therefore, values of blast-associated physical parameters are not uniform across the space disturbed by the event.
Aerodynamic drag, and more particularly, shock reflections off liquid and solid surfaces, generate a significant range of the above parameters within any explosion that occurs near the earth's surface or structures. All of these values change quickly due to the transient nature of blast effects.
Ideal Gas Models for Calculating Blast Wave Properties
For the foregoing reasons, approximations are often made using “ideal gas assumptions” for calculating values characterizing explosion phenomenology. Because of the range of parameter values and uncertainty in measuring these values in large explosions, calculations using ideal gas assumptions are generally adequate.
Ideal gas formulae are based upon relationships between measured pressure, temperature, and volume of numerous gases tested in experiments dating back to the nineteenth century. The mathematical linkage between these parameters applies from ambient atmospheric conditions (air density of approximately 1.169 kilograms per cubic meter and temperature of 25 degrees Celsius) to roughly 1,000 times ambient.
Beyond 1,000 bars, deviations from calculations made using ideal gas formulae are still less than 80% from actual values up to 400 bars and 300 degrees Kelvin. The use of a compressibility factor chosen from experimentally-derived diagrams enables use of ideal gas formulae to closely estimate gas properties at high temperatures and pressures.
By definition, shock waves propagate at velocities above the acoustic speed in the medium. Velocity of shock waves and objects traveling in air are often reported in terms of the Mach number or Mach speed M, defined as the ratio of velocity to the speed of sound (acoustic speed) a in the local medium. Using ideal gas assumptions, the following relationships apply for an isentropic process:P0/P=[1+M2(k−1)/2]k/(k−1) and p0/p=[1+M2(k−1)/2]1/k−1)where P0 and p0 are the pressure and density, respectively, of the ambient gas, P and p are respectively the pressure and density in the medium at a point in the moving gas stream, M is the Mach speed of the moving gas stream, and k is the ratio of the specific heats respectively at constant volume and pressure of the subject gas. Shock wave propagation is so rapid that the isentropic assumption is valid in most applications involving explosions.Acceleration of Gas Components
Shock waves accelerate atomic and molecular species comprising the gas medium to what is typically called the “particle velocity” or “blast wind”. The initial value of velocity of the accelerated molecules is defined as the particle velocity, designated up. Using ideal gas assumptions, the relationship between particle velocity and the acoustic speed in the ambient air isup/ax=5(Mx2−1)/6Mx where ax is the ambient-air acoustic speed and Mx the Mach speed of the moving air mass with respect to the ambient air.
For an explosive charges equivalent to approximately 10 to 20 kilograms of TNT (2, 4, 6-trinitrotoluene), accelerated hot gas will impinge upon surfaces separated between 0.5 and 1 meter from the charge at Mach speeds roughly between 5 and 12. Isentropic compression will increase gas density to roughly 5.5 times the ambient value. With heating to 1,600 degrees Celsius in quasi-static conditions, gas density in this space may increase to as much as 40 kilograms per cubic meter.
Also important to predicting blast parameters is consideration of shock waves reflecting from objects. Reflected shock waves propagate in gas that is made denser, hotter, and at greater pressure than present in the incident shock wave. Thus reflected shocks have faster velocities and generate much more destructive power than the incident shock wave.
In air at normal incidence, the ratio of reflected shock overpressure Pr to incident overpressure Px in a gas with specific heat ratio k of 1.4 (such as air) isPr/Px=(4Mx2−1)(7Mx2−1)/3(Mx2+5)where Mx is the Mach speed of the impinging shock wave. Reflected pressure can thus be as much as 8 times higher than for the blast wave impinging on a rigid surface. Advancing the art of blast protection for structures and vehicles requires substantial reduction of reflected shock parameters.Deflagrations Involving Flammable Dusts and Gases
For true explosives, propagation of the combustion reaction occurs due to pressure. Because shock wave peak pressure is sufficient to propagate combustion, actual detonation occurs in true explosives.
In contrast, combustion in flammable, non-condensed materials is propagated by heat transfer. As noted previously, such a combustion reaction is termed a deflagration. Unlike with solid explosive materials, scaled distance comparisons of different flammable gases and dusts cannot be made.
Mass of the reactants and products involved with non-condensed phase deflagrations is typically much lower than with detonating solid explosives. Thus the inertia of explosions arising from flammable mists and vapors is considerably lower than encountered with solid explosive detonations.
Overpressure developed by a deflagration is mathematically linked to the flame front velocity and temperature as it advances into the unburned flammable material. Explosions involving flammable dusts, mist, and vapors begin at relatively low velocities. Flame front velocity will increase rapidly as it evolves more hot, high-pressure combustion product gas.
Radiation from the flame front will preheat the unreacted material, which increases its flammability. The accelerating flame front will generate turbulence that facilitates combustion, as will obstacles encountered by the advancing flame front. Unless cooled, decelerated, or the flame front moves into unreacted material outside the flammability range (ratio of flammable material to oxygen), velocity of the flame front will produce a shock wave, e.g. a deflagration.
Blast Deflectors
Oblique reflected shock parameters are typically lower than for normal shock incidence. They also transfer momentum to the impinging blast wave so that a substantial portion of the accelerated gas is diverted outward from the loaded surface, thereby reducing QSP load. Protective barriers or armor configurations that avoid normal blast wave incidence are thus generally helpful for protecting objects behind them.
A combination of computer modeling and experiments led to development of a standard deflector geometry for the US Army that could better protect vehicles from detonating ground mines. This deflector incorporated a wedge that fit on the center of the vehicle underside, adjoining surfaces that were closer to parallel with the ground, and with another angle change for the outer ends of the deflector that sloped more sharply upward—but not as steeply as the sides of the central wedge. A standard kit for protecting US Army trucks was subsequently developed using this deflector geometry.
The standard kit used rigid steel plate to make these deflectors. Although an improvement over flat-floored vehicles with respect to reducing QSP, use of such hard material could not reduce reflected blast parameters. Rigid surfaces generate severe reflected shock in every case.
The above deflector kits are impervious to gas flow as well as rigid. Thus they fail to substantially dissipate energy through irreversible aerodynamic drag losses as is possible by using perforated plates or grilles. This principle is well known and, in fact, was exploited by the US Army for mitigating blast effect for above-ground storage of large munition stockpiles during the 1980's and 1990's. The term applied to this concept by the US Army is “vented suppressive shielding”.
Perforated deflectors would seemingly offer a solution to the problem of excessive quasi-static pressure. They are a solution for moderate and weak blasts, but mass flow rate in severe blast environments is so great that flow through holes will choke. At Mach 10, for example, the exit of a hole would need to be greater than 500 times the entrance diameter to avoid choked gas flow. Strong reflected shock parameters would still be produced, therefore, when choked flow conditions develop. Ground mines typically generate very severe blast conditions. Perforated deflectors made of conventional materials and with the present art would therefore be ineffective against most anti-armor ground mines.
Blast Parameters Requiring Mitigation
The greatest challenge to reducing the potential for harm from explosions is determining how to mitigate blast overpressure and impulse (momentum transfer). For protection of structures and vehicles against strong blasts, reducing impulse transmitted to and reflected from the object is most important.
Mitigating impulse requires that overpressure is strongly attenuated over the entire phase of blast loading. This is because duration of the blast load is much more difficult to reduce. In other words, reducing peak overpressure may not significantly affect impulse.
Indeed, one of the major shortcomings of the existing art is that mitigating materials and designs typically increase positive pressure duration. This allows quasi-static pressure (“QSP”, which is roughly constant pressure prior to venting or release of gas through failure of confining surfaces) conditions to develop in the presence of large exposed areas.
Shock waves traveling through gas compressed by a blast serve to further increase pressure. Reducing the time of loading by pressurized gas has heretofore been impossible to achieve when venting of the hot gas is inadequate.
In intense blast environments, the time scale of the high-pressure phase is typically longer than is needed for the object loaded by the blast wave to respond. This is particularly the case for vehicles attacked by ground mines and structures loaded by detonations of large explosive charges nearby. Wall accelerations and acceleration of whole vehicles in these events often inflict severe damage before blast effect dissipates into the surrounding environment.
Reducing pressure during blast loading requires mitigation of several blast-related phenomena. First, reflected shock must be attenuated. Reflected shock parameters dominate determination of total impulse imparted to the target since reflected pressure is almost always greater than incident. Duration of the reflected pressure phase is much longer than the incident phase when wide surface areas are presented to the blast. Second, one must also strive to deflect or divert hot gas around the target. This is to minimize quasi-static pressure (QSP). Third, one must prevent superposition of the shock wave reaching the target with the particle velocity wave, particularly that of the arrival of the hot gas just after formation of the reflected blast wave. Fourth, one can create irreversible energy losses through aerodynamic, viscous, and frictional losses.
Further reductions of blast impulse in outdoor environments or large spaces can be achieved if the protective assembly resists formation of a concave surface. A concave surface will trap hot gas at elevated reflected pressures, thereby adding to QSP.
Specific Problems with QSP
Numerous tests have proven that substantial attenuation of shock wave overpressure and impulse is achieved when media consisting of two phases in a granular or bead form are in close proximity to the source of the explosion. A significant range of two-phase attenuating media have demonstrated the effectiveness of this approach. Hollow ceramic beads, volcanic foam glass granules such as perlite and pumice, polystyrene foam beads, vermiculite, and similar media have all been successful in this regard.
Despite these successes, however, residual impulse from strong blasts has still been adequate to produce substantial accelerations and blast loads on structures presenting a large surface area to the explosion. Partially- and fully-confined explosions within containment substantially lined with two-phase blast-mitigating media have proven even more destructive except for charges smaller than threats typically posed by terrorists and military munitions. The problem in each of these environments is primarily that of quasi-static pressure associated with rapid generation of hot blast product gas that cannot be vented or diverted quickly enough.
Materials for Reducing Blast Damage
As noted above, almost all homogeneous materials used for mitigating blasts consist of two phases, typically solid and gas: Water barriers have also been evaluated many times, where rupture of the confinement releases water that is transformed into droplets by the transmitting blast wave.
Recently, metallic foams have been tested against blast loads based upon expectations that their collapse at relatively low pressure, their cellular structure, and variable acoustic speed would provide beneficial effects. So have zeolites for similar reasons, attempting to take advantage of their porosity and compressibility.
Despite vigorous efforts around the world, however, no homogeneous materials in the existing art have demonstrated the ability to adequately protect vehicles and ordinary buildings against severe blasts generated by detonations of large charges of solid explosives. For reasons more fully explained in the following section, existing materials have proven only able to mitigate some of the damaging mechanisms.
Generally these same mitigating materials can actually enhance damage through other physical mechanisms. This unfortunate phenomenon has been observed with water barriers, aluminum foam, honeycomb, polymeric foam, slit-foil spheroids, aqueous foam, and occasionally with panel assemblies filled with bead materials consisting of two phases such as perlite.
Shock Wave Propagation in Condensed Media
The foregoing discussion addressed blast phenomena in gases such as air. Pressurized hot gas produced by blasts may impinge on structures and vehicles. Fragments and projectiles accelerated by explosions may also strike structures and vehicles. These impacts must also be considered for blast protection design.
An empirical mathematical linkage between shock wave propagation in condensed media (solids, liquids, and gels) and the acoustic speed has been documented through decades of experiments, which isU=C0+su where U is the shock wave velocity, u is particle velocity, C0 is an empirical constant called the bulk acoustic speed and is the intercept of the U (vertical) axis on the U/u plane of a line drawn through the data plots, and s is the slope of this line. C0 and s are specific to the material through which the shock wave travels.
Values of s range from 0.9 for gases to 1.5 for most metals, and almost 2 for water. Values of C0 in metals range from 2.05 kilometers per second (km/s) for lead to greater than 5 km/s for aluminum alloys, around 0.9 for gases and 1.65 for water. Actual longitudinal sound speed (acoustic speed) is usually somewhat greater than Co, but is much less than double. Sound speed for aluminum, for example, is 6.4 km/s, compared with its C0 of 5.0-5.4 km/s. Although C0 is not the actual acoustic velocity (which is generally called the “longitudinal acoustic velocity”) of the material, it is linked to this physical parameter, generally being within 25% for most solid materials of commercial or military interest.
Shock wave pressures within materials are mathematically linked to density as well through the widely-used Bernoulli relationshipP1=p0C0(u1−u0)+p0s(u1−u0)2 where P1 is the pressure at and behind the shock wave front, u1 is the particle velocity behind the shock front, and uo is the particle velocity of the material in which the shock wave is traveling before its arrival (u0=0 for material at rest). For ranges of military interest, one can readily see that low density results in lower shock wave pressure. Particle velocities are limited by this relationship for ranges of military interest, since velocities of military projectiles, shaped-charge penetrators, and fragments from exploding munitions fall between 0.3 to roughly 8 kilometers/second (km/s). Values for s, C0 and p0 are even more constrained.
Density and shock wave transmission velocity are linked in yet another way, specifically through a parameter termed “impedance”. Impedance Z is defined as the mathematical product of density p and shock wave velocity U, orZ=pUAlthough density varies somewhat, impedance Z is essentially constant over ranges of values applicable to most problems of practical concern. Impedance is very important to mechanisms involved with projectile and high-velocity fragment impact damage.Shock Wave Propagation From One Material Into Another In Direct Contact
When shock waves travel through a material and reach a free surface (boundary with a lower-impedance medium), a rarefaction or relief wave will reflect back into the material. This rarefaction wave will have the same pressure as that of the low-impedance medium. When a shock wave transits any kind of material and reaches the interface with a solid material, what happens next is determined by the relative impedance of the 2 materials.
When a shock travels from a material having a higher impedance (Z) into a material of lower impedance, the shock wave will be reflected into the impinging medium and transmit into the impacted material as well. Pressure at the interface of impinging and impacting materials will decrease from its magnitude prior to reaching the interface. Following interaction at the interface between the 2 materials, particle velocity will increase in the impinging material compared to its value prior to the interaction. Shock wave velocity will be higher in the target material than in the impinging higher-impedance material.
The converse is true, also, meaning that a shock wave traveling through a low-impedance material into a material having a higher impedance will increase in pressure at the interface from its value just before reaching the interface. Particle velocity in the lower-impedance impinging material will decrease after interaction with the impacted material.
Significantly, particle velocities as well as shock pressure at interfaces must be equal. Also important is the fact that particle velocities double at interfaces between gases and condensed phases. These two facts have substantial ramifications for mitigation of quasi-static blast loading by hot gas at high pressure and for minimizing damage in armor materials impacted by projectiles.
Projectile Impact
When a projectile impacts a target having higher impedance, the shock wave reflected from the projectile/target interface transmits to the free surfaces at the sides and rear. At these surfaces, the shock wave reflects again, traveling through the projectile as a rarefaction or relief wave having the pressure of the surrounding medium, or ambient pressure. Upon reaching the target/projectile interface, this rarefaction wave is transmitted into the target. The two materials then are induced to separate unless held together in tension by strong bonding.
When the opposite case obtains, namely when a projectile strikes a target of lower impedance, a more complex series of events develops. Multiple shock wave reflections occur at the projectile/target interface. If both target and projectile are relatively short or thin, numerous reflections will develop between the target/projectile interface and the free surfaces. Each positive-pressure shock wave will transmit into the target material, although each successive shock wave will be weaker than the preceding one. Rarefaction or relief waves develop each time a positive-pressure shock wave reaches a free surface.
Should a material or assembly disintegrate during its interaction with a blast, conditions in the immediate vicinity of the shattered medium would be constrained by the shock pressure at that moment. Many new free surfaces would be created, and pressures at the numerous new interfaces between gas and shattered material would be the same. If shock pressure within the material is strongly reduced prior to disintegration, then pressure within the shattered components and the surrounding gas will be correspondingly low. Shock wave and particle velocities would be substantially reduced as well. If the shattered material or assembly was serving to isolate the environments on either side, then the reduced pressure on the blast side would be felt on the opposite side.
Ranges of Shock and Projectile Impact Parameters
The range of important properties of hot blast product gases must be considered in designing protective means. This is because most vehicles and structures exposed to blasts may be faced with a range of charge weights, explosive materials, and degrees of confinement.
For large ground mine detonations beneath vehicles, such as a 10-kg TNT charge at a spacing of 30 cm from the vehicle underside, multiple reflections of shock waves between vehicle and ground will occur. Gas density may exceed 30 kg per cubic meter at temperatures exceeding 1,500 degrees Kelvin. Peak pressure may exceed 2,000 bar. Duration of the positive overpressure will certainly exceed 100 milliseconds if detonation occurs near the center of the vehicle underside. Roughly similar conditions will prevail near a large wall impacted by a blast wave generated by a 5,000 kg TNT detonation 5 meters away.
Duration of shock wave propagation within solid components of protective assemblies is much shorter with projectile impacts. Armor layers are typically in the range of 6 mm to 60 mm for vehicle undersides and for protection of sides and top against automatic rifles and machine guns. Similar armor is used for protection against fragments produced by exploding artillery shells. A projectile or shock wave moving at 1 km/s travels 10 mm in 10 microseconds.
Gun-launched projectiles typically travel between 0.5 and 1.5 km/s. Artillery shell fragments near the bursting projectile travel between 1.3 and 3 km/s. This overlaps the range for explosively-formed penetrators (1.5-3 km/s). Particle velocities produced by projectile impacts and with layers within armor assemblies subjected to shock loading from contiguous layers typically range from 0.5 to 1 km/s. Thus one can see that high pressure durations associated with exposure to shock waves and projectiles are on the order of 1/10th that of blast load durations imposed by hot blast gases.
Peak and average pressures created by projectile impacts are much higher than overpressures from hot gas products generated by detonations. Peak overpressure from large explosive charge detonations beneath vehicles will be less than 1 GPa (10,000 bar). Peak impact pressure from EFPs may reach 40 GPa and 30 GPa for high-velocity fragments and gun-launched projectiles.
In contrast to condensed phase detonations, deflagrations involving dusts and gases produce much lower overpressures and slow shock waves. Peak overpressures greater than 8 bar are difficult to produce even in laboratory conditions. Chemical process facility deflagrations rarely exceed 2 bar. Durations, however, are typically very long, and can exceed 500 milliseconds.
Aerogels for Mitigation of Blast Effects
An opportunity now exists to provide protection against a wide range of explosive threats through an invention utilizing aerogel materials. Aerogels are described in many publications, with U.S. Pat. No. 6,989,123 filed by Kang P. Lee et al being a particularly useful source.
Aerogels have set records for lowest density of any solid ever produced and the lowest acoustic speed (70 meters per second). They have also established the record for highest specific surface area (1,200 square meters per gram). Features common to most aerogels developed to date are quite desirable in blast protection roles.
Although commercially marketed aerogels have densities comparable to conventional rigid foams (specific gravities ranging from 0.1 to 0.3), structural differences are pronounced. The nanostructure of aerogels features characteristic dimensions of cells less than the mean free path of gas molecules. Inhibiting intermolecular collisions through aerogel's nanostructure would dramatically reduce heat transfer.
Acoustic wave propagation is similarly made difficult by aerogel nanostructure, so that even with comparable density, acoustic speed and thermal conduction of conventional rigid foams are much higher than in aerogels. In this regard, aerogels offer unique advantages over the recently-proposed use of hydrophobic zeolite materials saturated in water under pressure.
Surprisingly, aerogels typically feature significant mechanical strength and tolerance for elevated temperatures. These qualities, in combination with low acoustic velocity and low density, make aerogels quite suitable for mitigation of blasts.
Aerogel products are generally too fragile to be used alone, but innovative arrangements with other components can be used to meet desired levels of protection with weights and thicknesses considerably lower than protective assemblies made with the current art. Many materials would be suitable for use in blast protection assemblies in combination with aerogels. In particular, metal foams can be incorporated to advantage in these arrangements as can other components in synergistic combinations as described subsequently.
Referring to the formulae presented above, one can readily see that the remarkably low longitudinal acoustic velocity of aerogels would strongly decelerate transmitting shock waves. This is because particle velocity u, shock wave velocity U, bulk acoustic speed C0, and actual (longitudinal) acoustic speed CL are of the same order of magnitude. The low density of aerogels would also greatly reduce transiting shock wave pressure due to the Bernoulli equation presented previously.
Since shock wave pressure and particle velocities must be equal at the interface between two materials in contact (such as between a projectile in contact with a target), aerogels potentially offer a means of strongly reducing shattering and plugging effects in target materials. The combination of reduced shock wave pressure and velocity would mitigate the environment around the blast or projectile impact on a target, even if the target is penetrated.
Blast protection possibilities with aerogels would apply to both normal and oblique blast wave impingement. The much-reduced reflected blast parameters would strongly attenuate Mach stem formation and propagation. Mach stem is the wave formed at low angles of blast wave impingement on surfaces by the combination of incident and reflected shock waves.
Aerogels thus theoretically offer advantages both for blast protection cladding of structures and for deflector assemblies. If designed and used properly, deflectors would theoretically benefit greatly from aerogel exteriors. This would occur due to the extra time before blast waves would transit the aerogel and reach the structure, thereby enabling more of the blast wave to be deflected away.
Aerogels and the Current Art for Blast Protection Armor
Using aerogels in the same manner that conventional cladding and deflector assemblies are presently used would undermine or negate their theoretical advantages. Most particularly, fragile aerogels would be exposed to a wide range of hazards. This approach would also fail to significantly reduce quasi-static pressure (QSP), since no heat transfer or significant aerodynamic drag losses would be produced.
Advantage of low reflected blast parameters would still obtain with aerogels used as cladding, but the very low shock wave and particle velocities would ensure superposition of incident and reflected shock waves when aerogel thickness exceeds 2 cm. This would result in increased impulse (momentum transfer) from the blast into the structure, even more than has been documented when aqueous and conventional solid foams have been similarly used.
Positive overpressure durations trapped in such layers would certainly persist for the durations typical of the intense blasts associated with ground mine detonations beneath vehicles and large charge detonations near sizable structures. Employment of thin aerogel layers would reduce duration of positive shock wave overpressures within the aerogel but would prevent the aerogel from substantially reducing blast pressure and velocity.
Expanded Metal Products for Blast Mitigation
Suppression of deflagrations has been demonstrated using cellular product forms that decelerate flame front velocity and extract heat from it. These products have appeared as reticulated foams and beads comprised of slit metal foil. Reticulated foams have been made from polymeric materials and by expanding slit aluminum foil into a flexible batt form. United States military specifications exist that cover both types of products. Both types are employed in many military aircraft to suppress catastrophic fuel tank explosions.
Examples of commercially-marketed, expanded slit-foil beads include products tradenamed Explosafe™ and Firexx™. The much higher heat transfer coefficient of aluminum foil in these products render them more capable of rapid heat extraction from hot deflagration gas than polymeric reticulated foam. Both forms of products decelerate flame fronts and shock waves.
Mixed success has been found with products of the above forms using the current art. In many cases, they have clearly been successful in preventing major fuel tank damage. This is particularly the case in electric spark-initiated deflagrations. Strong deflagrations generated by exploding incendiary projectiles, however, accelerate the reticulated materials and slit-foil beads. Inertial loads so generated in reticulated foams have been shown to be destructive to the walls of fuel tanks.
Firexx™ has demonstrated effectiveness in mitigating blasts from detonating solid explosives when a significant distance between the charge and metal bead layer exists. A noteworthy example is a US Government test in which an unreinforced concrete masonry wall was kept intact by a barrier of Firexx™ when exposed to a moderately intense blast (approximately 1 m/kg1/3 scaled distance). Blast product gas was unquestionably hot in this event when it encountered the Firexx™ barrier. The combination of aerodynamic drag energy loss from the blast wave, attenuation of reflected shock parameters, and rapid cooling during the QSP phase proved adequate for protecting this relatively weak wall. These characteristics are significant to development of blast mitigation assemblies.
A drawback to use of such materials is the substantial thickness required for them to mitigate blast parameters. Unlike aqueous foams and other two-phase cellular media, beads comprised of slit metal foils are poor acoustic and shock wave attenuators. Blast barriers must be at least 15 centimeters to effectively protect against blast intensities around 1 m/kg1/3, and thicker for scaled distances less than this. Containers and tanks must be mostly or completely filled in order to suppress blasts in fuel vapors. Many applications, such as containers and the underside of vehicles, do not have space to allow such thick protective barriers.
Metallic Foams
Metals can now be manufactured that have cellular or spongiform internal structures and solid surfaces. With the current art, the largest cells or void space is around the center, with decreasing porosity near the surfaces. Presently, metallic foam plates can be made having less than 50% of solid bulk density.
Aluminum has been the most popular metallic foam commercialized to date, but metal foams using other metals have been produced. Variable density and non-uniform cellular or spongiform internal structure offers possibilities of usefulness in disrupting gas flow at high velocity as it transmits into the interior of a metallic foam. In particular, the acoustic speed of solid aluminum is high, being more than 6,000 meters per second. Such a high acoustic speed would allow shock waves to propagate over a wide area along the surfaces of aluminum foam.
Increasing porosity and the spongiform internal structure would greatly reduce this acoustic speed in the middle of aluminum foam. Thus a shock wave generated either by projectile impact or intense blast wave impingement would distribute over a wide area transverse to the direction of shock wave propagation while propagation along the incident direction would be substantially reduced.
Frangible Materials
Frangible materials and components are those that shatter easily upon blast load incidence or impact. Very little energy is dissipated in this process but reflected shock wave intensity is greatly reduced compared with tough surfaces. Thin glass, for example, is frangible but thick glass plate is not.
Frangible surface components may serve to provide a washable surface or otherwise isolate the external environment from the opposite side. Within an assembly consisting of several layers, a frangible component may serve to confine or retain other components as well as to separate spaces.
Thin plastic sheets and rigid foam boards are frequently used as frangible components. This is because they have low mass and disintegrate quickly. However, they feature relatively low acoustic speeds and therefore cannot quickly redistribute shock waves transverse to the incident direction.
Blast wave parameters for the gas transmitting through the disintegrating component are at least as great as at the intact frangible surface. This facilitates intense, localized blast loading of the rear components and beyond.
Metals, with their inherently high acoustic speeds would thus be preferable as frangible elements. Their yield strength, mass, and ductility make them inappropriate, however, even when very thin. Because of their strength at low pressure, metals are typically used as rupture disks in safety equipment for the chemical process industry and as diaphragms in laboratory shock tubes.
Ceramic materials typically have acoustic speeds higher than metals, which is desirable. They also are generally amenable to shattering upon impact and blast pressure. However, their densities are typically very high and are generally more expensive than metals.
Metal foams would be preferable to solid metals because of their lower density. Stress and shock waves would travel quickly along the continuous surface layers while traveling much slower through the spongiform internal structure. Unless weakened in preferred patterns, however, metal foams would remain intact. Remaining intact would prevent the desired frangible behavior.
Frangibility can be introduced with all of these materials by bonding small pieces of each into sheets or other desired shapes. Ceramic pieces of tungsten carbide or alumina, for example, could be bonded by adhesives or resins and then formed as sheets. The same could be done with metal foam pieces, plastic and glass beads, and metals. This technique is within the current art.
Nozzles and Ducts
Energy losses are generated in gas flow in ducts, pipes, and nozzles at high mass flow rates. Friction along the walls increases as gas velocity increases. Unless properly designed, turbulence will also develop at high flow rates. For gas flow around the acoustic speed, complex, secondary shock phenomena will develop in ordinary ducts.
Maximum mass flow through a nozzle (a duct with a reduced area at one location) will occur at the acoustic speed of the gas medium. Ducts with constant cross sections cannot achieve as high a mass flow rate as can happen in proper nozzles with throats having the same cross section as the duct. Shock waves reflecting off the surfaces of imperfect nozzle walls and ordinary ducts generate complex, secondary shock phenomena. Turbulence ensues as a result, and mass flow rate is reduced from the theoretical maximum.
For intense blast loads near large surface areas, high mass flow rates of the impinging gas directed away from the surface are required in order to prevent unacceptable damage. This fact suggests that arrangements within assemblies intended to reduce blast loads on structures, container walls, and vehicles must perform as nozzles.